Abstract:

Longmenshan tectonic nappe belt is situated on the northwestern margin of Sichuan Basin,and is approximately 450km long and 40-80km wide,striking in northeast direction.This belt comprises variety of various size of nappes,nappe fragments and slices.These different size tectonic units are separated by diverse over-thrusts,which were developed along tectonically weakened bedding planes in the crust.It is inferred from depth of seismic focuses that the major decollement plane is lying about 17-22km beneath the surface.In addition,some second class of decollement planes are also found at the top of pre-Sinian crystaline basement and within Silurian,Permian and Lower Triassic rocks.The Longmenshan nappe belt was progressively developed from northwestto southeast in close relation with three successive collisions between Indian and Eurasian plates.Namely the mechanics which result in the formation of Long-menshan was exerted from the eastward compressional component of this col-lisions,or it may be presumed that the uplifting of the Qinghai-Tibet plateauand the thickening of the crust that gravity dispersion and the subsequent gravity sliding happened,leads to the formation of series of klippes in thefront of Longmenshan mountains.Typical reverse stratigraphical sequences can be noticed in the domain of this nappe belt.It is suggested that the underlying Sinian to Triassic rocks may contain oiland gas,because they were sedimented in the front of Yangtze para-platform,a condition favourable to oil and gas generation.